1. Field of the Invention
This invention relates to methods of and apparatus for video signal processing, and more particularly, but not exclusively, to methods of and apparatus for video signal processing in which temporal inter-frame processing is involved, such as in video signal to photographic film conversion.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows in block diagrammatic form an apparatus which we have previously proposed for video signal to photographic film conversion. The apparatus comprises a video tape recorder (VTR) 1, a video signal converter 2, a gamma corrector 3, and an electron beam recorder (EBR) 4 interconnected as shown. Briefly, the operation is as follows.
The VTR 1 reproduces a high definition video signal (HDVS) comprising 60 interlace scanned fields per second. The converter 2 is a relatively simple device which produces from the reproduced video signal a converted video signal corresponding to a 24 progressive scan frames per second rate, formed by dropping every fifth input field and combining the remaining input fields to form progressive scan frames, which are supplied to the electron beam recorder 4 to be recorded on photographic film.
A problem with this apparatus is that if there is significant movement in a scene, then the conversion process used results in a very obtrusive 6 Hz motion judder component in the picture.
More sophisticated video signal converters are known in the form of video standards converters used to convert video signals from one television standard to another, for example, from a 625 lines per frame, 50 fields per second standard to a 525 lines per frame, 60 fields per second standard. Video standards conversion cannot be achieved satisfactorily merely by using simple linear interpolation techniques, because of the temporal and vertical alias which is present in a video signal. Thus, simple linear interpolation produces unwanted artefacts in the resulting picture, in particular, the pictures are blurred vertically and judder temporally.
To reduce these problems it has been proposed that video standards converters should use adaptive techniques to switch the parameters of a linear interpolator in dependence on the degree of movement in the picture represented by the incoming video signal.
It has also been proposed, for example for the purpose of data reduction in video signal processing, to generate motion vectors from an incoming video signal by a block matching technique, in which the content of a search block in one field or frame is compared with the respective contents of a plurality of search blocks comprised in a search area in the following field or frame, to determine the minimum difference between the contents so compared, and hence the direction and distance of motion (if any) of the content of the original search block.
In Sony Corporation U.S. Pat. No. 5,016,101 there is disclosed apparatus for converting a video signal to a photographic film image, and incorporating a motion compensated television standards converter. A problem with this apparatus, and which occurs generally in motion adaptive and motion compensated video signal processing methods and apparatus, is that in order to drive the signal processing accurately, the information derived about the motion of objects in the image must be accurate. A particular difficulty is that scene changes, and indeed cuts generally, wipes, fades and some other effects, appear to the detection circuitry as rapid random changes in motion. This can cause unpleasant artifacts in the output image, which may be even more noticeable if any post-processing is applied to the video signal.